BLAST Search main parameters

The statistical significance threshold for reporting
matches against database sequences; the default value
is 10 in NCBI-BLAST, such that 10 matches are expected to be found
merely by chance, according to the stochastic model
of Karlin and Altschul (1990). If the statistical
significance ascribed to a match is greater than the
EXPECT threshold, the match will not be reported.
Lower EXPECT thresholds are more stringent, leading
to fewer chance matches being reported. Fractional
values are acceptable.

In our program, we set the default E-value 1e-5. The lower of the E-value is chosen, the more accurate the prediction will be.